1. Field of the Invention
The present invention relates to a digital type VSB modulation apparatus for generating a VSB (vestigial sideband) modulation signal from a digital baseband signal.
2. Description of the Related Art
As is well known, a video modulation means used in a conventional television transmission apparatus AM-modulates an analog baseband signal to convert it into a DSB (double sideband) modulation signal, and limits the bandwidth of the DSB modulation signal using a band-pass filter included in an intermediate frequency circuit or a transmission frequency circuit, thereby obtaining a VSB modulation signal (see, e.g., "Broadcast Technology Library 10, Broadcast Equipment, Chapter 3, Television Broadcast Equipment" issued on Apr. 20, 1983 by Nippon Hoso Publishing Inc.). In this case, as the band-pass filter, a surface acoustic filter is generally used. However, the surface acoustic filter is economically disadvantageous since separate filters must be prepared in units of use frequencies. In addition, the surface acoustic filter suffers from large ripples of group delay time characteristics due to its structure. For this reason, the group delay time characteristics of a transmission apparatus itself depend on the surface acoustic filter.
Recently, strong demand has arisen for a digital VSB modulation apparatus. However, it is very difficult in terms of a structure to simply replace circuit sections constituting a conventional VSB modulation means with digital circuits having the same functions. For example, when the band-pass filter is realized by a digital filter, a high-speed circuit operation is required, and the number of taps becomes very large since the attenuation characteristic curve of the stop band is very steep, resulting in a large circuit scale.